Ordered arrays of thin filaments (65 A diameter) along with other apparently random arrangements of thin and thick filaments (100–200 A diameter) are observed in contracted guinea pig taenia coli rapidly fixed in glutaraldehyde. The thin-filament arrays vary from a few to more than 100 filaments in each array. The arrays are scattered among isolated thin and thick filaments. Some arrays are regular such as hexagonal; other arrays tend to be circular. However, few examples of rosettes with regular arrangements of thin filaments surrounding thick filaments are seen. Optical transforms of electron micrographs of thin-filament arrays give a nearest-neighbor spacing of the thin filaments in agreement with the "actin" filament spacing from x-ray diffraction experiments. Many thick filaments are closely associated with thin-filament arrays. Some thick filaments are hollow circles, although triangular shapes are also found. Thin-filament arrays and thick filaments extend into the cell for distances of at least a micron. Partially relaxed taenia coli shows thin-filament arrays but few thick filaments. The suggestion that thick filaments aggregate prior to contraction and disaggregate during relaxation is promoted by these observations. The results suggest that a sliding filament mechanism operates in smooth muscle as well as in striated muscle.
THE ORGANIZATION OF CONTRACTILE FILAMENTS IN A MAMMALIAN SMOOTH MUSCLE
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Robert V. Rice, Joan A. Moses, G. M. McManus, Arlene C. Brady, Lorraine M. Blasik; THE ORGANIZATION OF CONTRACTILE FILAMENTS IN A MAMMALIAN SMOOTH MUSCLE . J Cell Biol 1 October 1970; 47 (1): 183–196. doi: https://doi.org/10.1083/jcb.47.1.183
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